Bond cleavage esters

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Thermal decomposition of dihydroperoxides results in initial homolysis of an oxygen—oxygen bond foUowed by carbon—oxygen and carbon—carbon bond cleavages to yield mixtures of carbonyl compounds (ketones, aldehydes), esters, carboxyHc acids, hydrocarbons, and hydrogen peroxide. 

Other examples of the successful displacement of tosylates are the preparation of 31 -, 16a-,16j - and27- labeled steroids. This displacement reaction fails, however, with certain C-18 and C-19 alcohol derivatives which give mainly O—S instead of C—O bond cleavage. Unsatisfactory results were also obtained with sterically hindered tosylate esters at C-11, C-12 and C-20, which give considerable amounts of olefinic products in addition to O—S bond cleavage. ... 

A photochemical variant, the so-called photo-Fries rearrangement, proceeds via intermediate formation of radical species. Upon irradiation the phenyl ester molecules (1) are promoted into an excited state 11. By homolytic bond cleavage the radical-pair 12 is formed that reacts to the semiquinone 13, which in turn tautomerizes to the p-acylphenol 3. The corresponding ort/zo-derivative is formed in an analogous way ... 

Intramolecular and intermolecular 1,3-dipolar cycloadditions of aziridine-2-car-boxylic esters with alkenes and alkynes have been investigated [131, 132]. Upon heating, aziridine-2-carboxylates undergo C-2-C-3 bond cleavage to form azome-... 

Furthermore, Cordes etal995 observed the saturation-type kinetics, strongly suggesting the formation of a complex between the polyelectrolyte and ester preceding bond cleavage reactions, as has been found for micellar catalysis11,1015. 

In general, aliphatic diacyl peroxide initiators should be considered as sources of alkyl, rather than of aeyloxy radicals. With few exceptions, aliphatic acyloxy radicals have a transient existence at best. For certain diacyl peroxides (36) where R is a secondary or tertiary alkyl group there is controversy as to whether loss of carbon dioxide occurs in concert with 0-0 bond cleavage. Thus, ester end groups observed in polymers prepared with aliphatic diaeyl peroxides are unlikely to arise directly from initiation, but rather from transfer to initiator (see 3.3,2.1.4),... 

Both schemes accommodate the kinetics, the primary isotope effect and the induction factor, which indicates that Cr(IV) is the initial stage of reduction of the oxidant. RoCek s mechanism does not accord with the solvent isotope effect of 2.5 per proton, which has just the value to be expected for acid-catalysis, for the O-H bond cleavage should be subject to a primary isotope effect of about 7. The ester mechanism is not open to this criticim. 

There are also examples in which phosphate esters of saturated alcohols are reductively deoxygenated.229 Mechanistic studies of the cleavage of aryl dialkyl phosphates have indicated that the crucial C-O bond cleavage occurs after transfer of two electrons.230... 

Heating of oxyprotoberberine 55, derived from dehydroprotoberberine (54), in methanol saturated with potassium hydroxide effected C-8—N bond cleavage to afford the zwitterionic salt 56, which was converted to amino ester 57 (Scheme 13) (50). The same reaction, however, could not be performed for oxyberberine (58), which lacks the additional double bond in ring B. 

Three of the alkaloids (55-57) were isolated from Corydalis incisa (63-65) and two (57, 58) from Hypecoum procumbens (66), where they coexist with the parent protoberberines. They are formed in plants by an oxidative C-6—N bond cleavage, possibly through an aldehyde intermediate 61. This assumption was supported by in vivo experiments in which ( )-tetrahydrocorysamine-8,/4-/2 (59) and (+ )-tetrahydrocoptisine-8,74-/2 (60) were fed to Corydalis incisa (67) to produce corydalic acid methyl ester-8-/ (55) along with corydamine-8-f (58) and corydalic acid methyl ester-8-/ (55) with corynoline-8-/ (62), a benzophenantridine alkaloid, respectively (Scheme 14). 

A kinetic isotope effect 160/180 of 2% in the spontaneous hydrolysis of the 2,4-dinitrophenyl phosphate dianion, whose ester oxygen is labeled, suggests a P/O bond cleavage in the transition state of the reaction, and thus also constitutes compelling evidence for formation of the metaphosphate 66,67). The hydrolysis behavior of some phosphoro-thioates (110) is entirely analogous 68). 

The bonds between an oxygen and an sp3 carbon atom in alcohols, ethers, or esters are quite resistant to hydrogenolysis. Elevated temperatures and pressures are required to induce C-O bond cleavage and the high temperature can cause the cleavage of the C—C bonds, too. 

The formation of 2H-pyrroles (21) and a pyrrole derivative (22) from the reaction of 3-phenyl-2//-azirines and acetylenic esters in the presence of molybdenum hexacarbonyl is intriguing mechanistically (Schemes 24, 25).53 Carbon-nitrogen bond cleavage must occur perhaps via a molybdenum complex (cf. 23 in Scheme 26) but intermediate organometallic species have not yet been isolated.53 Despite the relatively poor yields of 2H-pyrrole products, the process is synthetically valuable since the equivalent uncatalyzed photochemical process produces isomeric 2H-pyrroles from a primary reaction of azirine C—C cleavage54 (Scheme 24). 

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